arXiv subjects

Maurizio Salaris

Publications and source records attributed to Maurizio Salaris.

96 records · Page 6Linked to original sources

arXiv2000

Population effects on the Red Giant Clump absolute magnitude, and distance determinations to nearby galaxies

(Abridged) The red giant clump has been recently argued to be a reliable distance indicator for the galaxies in the Local Group. We re-address the problem of the systematic magnitude differences (DeltaM_I) between the local clump revealed by Hipparcos, and the clump stars observed in distant galaxies. First, we present tables and equations with the theoretically-predicted clump M_I as a function of age and metallicity, that allow the computation of DeltaM_I for any given galaxy model. We then explain in some detail what determines the distribution of masses, ages, and metallicities of clump stars in a galaxy. We point out that in galaxies with recent/ongoing star formation, the age distribution of clump stars is strongly biased toward younger (1-3 Gyr) ages, and hence toward higher metallicities. Obviously, this does not happen in galaxies with predominantly old stellar populations. We construct a model for the local clump that is shown to produce distributions of metallicities, colours, and magnitudes, similar to those derived from spectroscopic and Hipparcos data. Bulge, Magellanic Clouds, and Carina dSph models are used to analyse the values of DeltaM_I for these systems. We show how the clump-RR Lyrae data is well reproduced by the models. However, despite this, the models indicate that the linear DeltaM_I vs. [Fe/H] relations that have been derived from the data are not general. Finally, we provide revised clump distances to the Bulge, Magellanic Clouds, and Carina dSph. The largest DeltaM_I values are found for the Magellanic Clouds and Carina dSph, which turn out to be located at distance moduli 0.2-0.3 mag longer than indicated by works which ignore population effects. The Galactic Bulge, instead, may be slightly closer (0.1 mag) than indicated by previous works based on the red clump.

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arXiv1999

HST observations of the LMC field around SN87A: distance determinations with Red Clump and Tip of the Red Giant Branch stars

We have used HST-WFPC2 multiband observations of a field around SN 1987A in the Large Magellanic Cloud to measure its distance from the Sun. The observations allowed us to carefully determine the interstellar extinction along the line of sight to a large number of stars and to measure the LMC distance by using two stellar distance indicators: the Red Clump and the Tip of the Red Giant Branch. From an application of the Red Clump method we obtain a distance modulus (m-M)o,rc(LMC)=18.59+-0.04+-0.08 mag (statistical plus systematic error), in good agreement with the distance derived by using the Tip of the Red Giant Branch stars, namely (m-M)o,trgb(LMC)=18.69+-0.25+-0.06 mag (statistical plus systematic error). Both values agree well with the distance to the SN 1987A as determined from a study of its inner ring fluorescent echo ((m-M)(SN87A)=18.55+-0.05 mag, Panagia 1998), thus excluding distance moduli lower than 18.43 to a 99.7% significance level. Differences with respect to previous results obtained using the same distance indicators are discussed.

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arXiv1999

Colour transformations for isochrones in the VI-plane

Reliable temperature-colour transformations are a necessary ingredient for isochrones to be compared with observed colour-magnitude-diagrams of globular clusters. We show that both theoretical and empirical published transformations to (V-I) to a large extend exhibit significant differences between them. Based on these comparisons we argue for particular transformations for dwarfs and giants to be preferred. We then show that our selected combination of transformations results in fits of V-(V-I)-CMDs of a quality which is comparable to that of our earlier V-(B-V) isochrones for a wide range of cluster metallicities. The cluster parameters, such as reddening, are consistent with those derived in (B-V). Therefore, at least in the case of the fit with our own isochrones - based on the particular distance scale provided by our own horizontal branch models, and on the treatment of convection by the mixing-length theory having l/H_p calibrated on our solar model - the chosen transformations appear to lead to self-consistent (V-I) isochrones. Our isochrones are now well tested and self-consistent for B, V and I photometric data.

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arXiv1999

Stellar Envelope Convection calibrated by Radiation Hydrodynamics Simulations: Influence on Globular Clusters Isochrones

One of the largest sources of uncertainty in the computation of globular cluster isochrones and hence in the age determination of globular clusters is the lack of a rigorous description of convection. Therefore, we calibrated the superadiabatic temperature gradient in the envelope of metal-poor low-mass stars according to the results from a new grid of 2D hydrodynamical models, which cover the Main Sequence and the lower Red Giant Branch of globular cluster stars. In practice, we still use for computing the evolutionary stellar models the traditional mixing length formalism, but we fix the mixing length parameter in order to reproduce the run of the entropy of the deeper adiabatic region of the stellar envelopes with effective temperature and gravity as obtained from the hydro-models. The detailed behaviour of the calibrated mixing length depends in a non-trivial way on the effective temperature, gravity and metallicity of the star. Nevertheless, the resulting isochrones for the relevant age range of galactic globular clusters have only small differences with respect to isochrones computed adopting a constant solar calibrated value of the mixing length. Accordingly, the age of globular clusters is reduced by 0.2 Gyr at most.

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arXiv1998

Metal-rich globular clusters in the galactic disk: new age determinations and the relation to halo clusters

New age determinations of the galactic disk globular clusters 47 Tuc, M71 and NGC6352 have been performed with our up-to-date alpha-enhanced stellar models. We find that all three clusters are about 9.2 Gyr old and therefore coeval with the oldest disk white dwarfs. Several arguments are presented which indicate that the initial helium content of the stars populating these clusters is close to the solar one. We also revisit a total of 28 halo clusters, for which we use an updated [Fe/H] scale. This new metallicity scale leads on average to an age reduction of around 0.8 Gyr relative to our previous results. We compare the predicted cluster distances, which result from our dating method, with the most recent distances based on HIPPARCOS parallaxes of local subdwarfs. We further demonstrate that for the most metal-rich clusters scaled-solar isochrones no longer can be used to replace alpha-enhanced ones at the same total metallicity. The implications of the presented age determinations are discussed in the context of the formation history of the Galaxy.

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arXiv1997

Chronology of the halo globular cluster system formation

Using up-to-date stellar models and isochrones we determine the age of 25 galactic halo clusters. The clusters are distributed into four groups according to metallicity. We measure the absolute age of a reference cluster in each group, and then find the relative ages of the other clusters relative to this one. This combination yields the most reliable results. We find that the oldest cluster group on average is 11.8+-0.9 Gyr or 12.3+-0.3 Gyr old, depending on whether we include Arp2 and Rup106. The average age of all clusters is about 10.5 Gyr. Questions concerning a common age for all clusters and a relation between metallicity and age are addressed. The groups of lower metallicity appear to be coeval, but our results indicate that globally the sample has an age spread without a simple age-metallicity relation.

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